1. Field of the Invention
The present invention relates to an optical deflector, a method of manufacturing the optical deflector, an optical scanning apparatus, and an image forming apparatus.
2. Description of the Related Art
Recent electrophotographic image forming apparatuses such as copiers, laser beam printers, and facsimile apparatuses convert electronic information into optical information, fix the optical information on a photoreceptor as latent images with an optical scanning apparatus, and develop the fixed latent images by toner or the like to form images. The optical scanning apparatus is composed of an optical system including a laser optical source, a cylindrical lens, a rotary polygon mirror, an image forming lens, a mirror, and the like. In the optical scanning apparatus, the laser beam from the laser optical source is formed into a luminous flux by the cylindrical lens, deflected for scanning by the rotary polygon mirror, and the deflected optical light is formed into images on the photoreceptor by the image forming lens, the mirror, and the like to form electrostatic latent images. In recent years and continuing to the present, such image forming apparatuses have achieved high speed and high density, and the rotation speed of the rotary polygon mirror of the optical scanning apparatus also has been greatly increased. Among the image forming apparatuses, color image forming machines generally need four colors to form images and optical scanning for each color. Therefore, the ones equipped with plural rotary polygon mirrors are used. Generally, the plural rotary polygon mirrors are laminated one on the other and fixed onto a rotary shaft to coaxially rotate, and are structured in such a manner as to reflect different incident lights at different positions to perform an optical scanning. Furthermore, in order to correspond to the high density in addition to the high speed of the image forming apparatuses, the tendency has been toward an increase in the rotational speed and the area of the reflection surface of the rotary polygon mirror. An increase in the rotational speed and the surface area of the rotary polygon mirror would inevitably cause vibrations, noise, increased wind noise due to the friction with ambient air, or a temperature rise due to an eccentricity or the like of the rotary polygon mirror.
In order to solve the problem of wind noise, in particular, there have been disclosed many documents. In Patent Document 1, for example, there is proposed an apparatus in which the surroundings of the rotary polygon mirror are covered, a transparent member is disposed at the area through which an optical beam passes, and an end of the transparent member is positioned at an inner wall surface near an opening edge on the downstream side in the rotating direction of the rotary polygon mirror. Accordingly, the apparatus prevents airflow, occurring when the rotary polygon mirror is rotated, from colliding against the inner wall surface near the opening edge on the downstream side in the rotating direction and the occurrence of turbulent flow caused by the collision to maintain laminar flow, thereby reducing the occurrence of noise. Patent Document 2 discloses an apparatus in which the reflection surfaces of first and second rotary polygon mirrors are shifted from each other in the rotating direction to allow reduction of wind noise. Patent Document 3 discloses an apparatus in which a protrusion is provided near the doorway of a deflector cover where an optical beam goes in and out. Accordingly, the apparatus generates the disturbance in an air flow to prevent the occurrence of the so-called whistle noise. Patent Document 4 discloses an apparatus in which an adhesive is applied to a concave portion for adjusting balance so as to correspond to the high-speed rotation of the rotary polygon mirror, thereby preventing the noise caused by the high-speed rotation. Patent Document 5 discloses improvement in a method of molding a resinous rotary polygon mirror, thereby preventing irregular rotation at the high-speed rotation and noise. Patent Document 6 discloses an optical deflector for a color image forming apparatus in which plural polygon mirrors are laminated in the rotary shaft direction and fixed together with deflection reflective surfaces of the polygon mirrors of each stage shifted from the others by a prescribed angle in the rotating direction.
In the above optical deflector, a rotating body is manufactured in such a manner that the plural rotary polygon mirrors are laminated one on the other and fitted on the rotary shaft to be assembled. In this case, it is necessary to use a highly accurate manufacturing technique to precisely adjust and fix the reflection surfaces of the plural rotary polygon mirrors by a prescribed angle to balance the rotating body to reduce vibrations or noise caused by the deflection surfaces at high-speed rotation. Moreover, there is a possibility of the reflection surfaces of the plural rotary polygon mirrors being shifted from each other due to acceleration and heat stress caused by their high-speed rotation or stopping operations.
Patent Document 1: JP-A-8-146330
Patent Document 2: JP-A-2003-315720
Patent Document 3: JP-B2-3489370
Patent Document 4: JP-A-2002-267988
Patent Document 5: JP-A-9-96711
Patent Document 6: JP-A-2005-92129
The present invention was made in view of the above problems and may provide a high-speed-and-high-density-ready optical deflector in which plural rotary polygon mirrors are firmly joined to each other and by which no rotational fluctuations are produced even at high-speed rotation, and a shift in position of the rotary polygon mirrors hardly takes place due to acceleration and temperature change; a method of manufacturing the optical deflector; an optical scanning apparatus using the optical deflector; and an image forming apparatus.